It is clear that both sides have shown greater flexibility, narrowed differences, and established a better atmosphere in which to negotiate difficult details. The hardest part may prove to be Iran's demand that the US acknowledge having committed "crimes" in Iran, including the CIA's well- documented involvement in the 1953 coup, which placed the Shah in power.

"That's ancient history . . .," declared President Carter when asked at a news conference Feb. 13 whether he thought it was proper for the US to restore the Shah to the throne against the popular will in Iran.

The President explained that he did not think it "appropriate or helpful . . . to go into the propriety of something that happened 30 years ago."

Another official later explained that the United States is not in a position even to consider answering to something that happened so long ago. He noted that the US was engaged in the cold war in the early 1950s and that what may appear to have been wrong from today's perspective might have appeared right and proper at the time.

One problem is that the Iranians are not only concerned with what happened 30 years ago, but also with what happened more recently under the Shah's secret police. They are convinced that the US was aware that the police were torturing political prisoners. Indeed, a recent US State Department report on human rights practices in Iran goes a long way toward admitting this. It states that "during the more than two decades of the Shah's rule, many thousands of Iranians were imprisoned for political reasons, and a significant number reportedly were tortured."

The US apparently would like to see the question of American responsibility or "guilt" resolved within the framework of a United Nations-sponsored commission of inquiry whose findings would not in any way be binding on the US. But even here President Carter was careful at his news conference to qualify his endorsement of the commission idea by saying that it would have to be "an appropriate commission with a carefully defined purpose" consistent with American goals and principles.

Most observers seem to think that for domestic political reasons alone President Carter is not likely to go much further than this in admitting to any American "guilt" for the misdeeds of the ousted Shah. What might be possible, some say, is a formula whereby the commission idea is combined with a vague, but public, American recognition of some degree of legitimacy to Iran's grievances against the Shah.

According to press reports from Tehran, Ayatollah Ruhollah Khomeini has approved a three-part plan that would call not only for a commission of inquiry and an admission of guilt on the part of the Americans but also for a process whereby assets taken by the Shah from Iran could be returned to that country.

This latter point might be the easiest to resolve. The US already has made clear in public statements that the American court system is open to Iranian claims to assets of the Shah and that the US would not oppose court action initiated by Iran. Some scholars familiar with Iranian thinking say a purely symbolic result in this regard might satisfy the Iranians. Other sources say the Shah's dollar holdings have probably been "laundered" through so many channels that they would be impossible to seize under any circumstances.

In the meantime, a new indication of improvement in the negotiating atmosphere came from Tehran in an interview that Iranian President Abolhassan Bani-Sadr gave to the Italian Television Network, RAI. According to United Press International, Mr. Bani-Sadr declared that "We depend on the United States for spare parts. They are essential to us."

This was a far cry from the atmosphere of just a few months ago, when Ayatollah Khomeini was saying that Iran did not need the United States, whereas the United States did need Iran. It also amounted to a hint that a new and cooperative relationship between Iran and the US might be possible once the hostage question is resolved.